1. Field of the Invention
The present invention relates to a variable magnification optical system which guides light from a photographing target (on the object side) to an image sensor, and an image-taking apparatus provided with such a variable magnification optical system. More specifically, the present invention relates to a variable magnification optical system provided with vibration-proof function and an image-taking apparatus provided with such a variable magnification optical system.
2. Description of the Related Arts
Conventionally, a variable magnification optical system having power arrangement, from the object side to the image side, “negative, positive, negative, and positive” is said to be suitable for achieving a wider angle and higher magnification. Thus, various variable magnification optical systems having such power arrangement have been disclosed (patent documents 1 to 4).
For example, patents documents 1 to 3 disclose variable magnification optical systems adopted for interchangeable lenses for a single-reflex camera or the like. In particular, these variable magnification optical systems achieve vibration-proof function (so called camera-shake correction function) by moving a lens group located at some position from the object side to the image side in the in-surface direction vertical to the optical axis direction.
Specifically, the camera-shake correction is performed by part of a second lens group of the variable magnification optical system of patent document 1, a third lens group of the variable magnification optical system of patent document 2, a latter half portion of the divided fourth lens group of the variable magnification optical system of patent document 3.
On the other hand, the variable magnification optical system of patent document 4 bends the optical axis by having an optical prism included in a most object side lens group. Thus, such a variable magnification optical system does not extend in one direction, and thus is easily loaded in a small-size image-taking apparatus (compact camera or the like).                [Patent document 1] JP-A-2004-61910        [Patent document 2] JP-A-11-174329        [Patent document 3] JP-A-2004-61679        [Patent document 3] JP-A-2004-205796        
The variable magnification optical systems disclosed in patent documents 1 to 3 are assumed to be used for an interchangeable lens for a single-reflex camera or the like. However, these variable magnification optical systems cannot provide sufficient aberration performance in compact digital cameras of a six-million-pixel class which have been recently distributed. Moreover, in zooming performed in a variable magnification optical system having “negative, positive, negative, and positive” optical powers for use in a single-lens reflex, a first lens group moves by utilizing a cam ring or the like. However, in a compact digital camera, the first lens group is moved directly by a driving force such as provided by an actuator or the like. Thus, it is preferable that the first lens group relatively heavy in weight be not moved.
What is important in a variable magnification optical system having a lens group which moves for camera-shake correction (vibration-proof lens group) is the degree of sensitivity to decentering, i.e., the ratio of the amount of movement of the vibration-proof lens group with respect to the amount of movement of an imaging point on the image surface. That is, the degree of sensitivity to decentering needs to be set appropriately. However, the variable magnification optical systems of patent documents 2 and 3 perform camera-shake correction by moving the lens group located relatively close to the image side (third or fourth lens group).
Such a case may bring about situation in which the degree of sensitivity to decentering is insufficient, but these variable magnification optical systems set the degree of sensitivity to decentering in just proportion by providing long back focus. Consequently, in a case of a small-size image-taking apparatus which inevitably has relatively short back focus, the variable magnification optical systems of patent documents 2 and 3 have difficulty in appropriately setting the degree of sensitivity to decentering. Therefore, the variable magnification optical systems of patent documents 2 and 3 are not suitable for being loaded in a small-size image-taking apparatus.
On the other hand, the variable magnification optical system of patent document 1 moves a partial portion in the single lens group in the in-surface direction vertical to the optical axis direction, thus permitting camera-shake correction by using a motor or the like which provides a relatively small thrust force. Thus, such a variable magnification optical system is preferably suitable for being loaded in a small-size image-taking apparatus. However, the lens groups of the variable magnification optical system are so designed as to be capable of correcting various aberration. Thus, in a case where camera-shake correction is performed by moving part of the single lens group, it is difficult to sufficiently correct the various aberration. Therefore, such a variable magnification optical system is capable of performing camera-shake correction but fails to provide a high-grade image (fail to sufficiently suppress various aberration in camera-shake correction).
The variable magnification optical system of patent document 4 is easy to be loaded in a small-size image-taking apparatus, but has no camera-shake correction function. Thus, the variable magnification optical system of patent document 4 is susceptible to camera-shake.